Here is how thw law reads in California:
•If boating on a vessel that is 26 feet or less, every child under 13 years of age must wear a PFD, unless in an enclosed cabin OR restrained by a harness tethered to a sailboat. Children's PFDs must be U.S. Coast Guard-approved of Type I, II, III or V and must be appropriate for wearer's size and weight. Inflatable PFDs are not designed for use by children under the age of 16. A violation of the child PFD law can result in a fine up to $250.
•Every person on board a personal watercraft (PWC) and any person being towed behind a vessel must wear a U.S. Coast Guard-approved Type I, II, III or V PFD. Ski belts do not meet this requirement and may be carried aboard as excess equipment only. Inflatable PFDs are not intended for use while participating in tow sports and are not permitted for this purpose.